Numerous methods have been developed for controlling the etch depth of the various constituent material layers during the fabrication of microelectronic devices, optoelectronic devices such as light-emitting and laser diodes, and micro-electro-mechanical systems. In order to etch a material down to a given depth, the etch can be terminated after a pre-determined amount of time using a calibrated etch rate.
However, run-to-run variation in the etch rate, layer thicknesses, and etch chamber uniformity can make it very difficult to consistently etch to a specified depth or vertical position in the layer structure over a full wafer.
Wet chemical etching many III-nitride materials is difficult due to very slow etch rates or the need for electrodes and illumination as in photoelectrochemical (PEC) etching. Dry etching processes, such as reactive ion etching (RIE) and inductively coupled plasma (ICP) RIE, are preferred for many applications over wet chemical etching because they can produce vertical sidewall structures and etch quickly through a wide variety of materials.
Selective etching makes use of varying etch rates among different materials, and is used in conjunction with a low-etch-rate etch-stop layer to greatly increase the process window, which results in a highly repeatable etch depth. An etch-stop layer having high etch selectivity with respect to material layers of AlN or high-Al-fraction AlGaN and InAlN is desirable to improve process control and reproducibility in ultra-wide-bandgap electronics, acoustoelectric devices, and UV optoelectronics. A candidate etch-stop layer should have high selectivity across a range of process conditions so that the dry etch process can be optimized to improve etch rate and anisotropy without sacrificing selectivity. No current etch-stop layer material fulfills these requirements for AlN or high Al-fraction AlGaN and InAlN.
Selective dry etching of GaN using an aluminum (Al)-containing etch-stop layer such as AlxGa1-xN or AlN is possible with BCl3/SF6 or Cl2/O2 etch chemistries. See D. Buttari, et al., “Selective Dry Etching of GaN Over AlGaN in BCl3/SF6 Mixtures,” International Journal of High Speed Electronics and Systems, vol. 14, pp. 756-761, 2004; and J.-M. Lee, et al., “Highly selective dry etching of III nitrides using an inductively coupled Cl2/Ar/O2 plasma,” J. Vac. Sci. Technol., B, vol. 18, pp. 1409-1411, 2000. However, the etch byproducts have very low vapor pressure at the etch temperature (e.g. <10−26 Torr for AlF3) relative to the process pressure (˜10−3 Torr), causing the etch byproducts to remain on the etch-stop layer surface and prevent further reaction from occurring.
The plots in FIG. 1 show the partial pressures of common RIE reaction byproducts at various temperatures for III-nitride materials. See G. H. Rinehart et al., “Vapor-Pressure And Vaporization Thermodynamics Of Scandium Trifluoride,” J. Less Common Met., vol. 75, pp. 65-78, 1980; and C. L. Yaws, The Yaws Handbook of Vapor Pressure: Antoine coefficients, 2nd ed. Waltham, Mass., USA: Gulf Professional Publishing, 2015, pp. 316-320. Indium Chloride (InCl3) (shown by plotline 107 in FIG. 1) is the dominant reaction product in RIE processes, see S. J. Pearton, et al., “A Review of Dry Etching of GaN and Related Materials,” MRS Internet J. Nitride Semicond. Res., 5, 11, pp. 1-38 (2000), and as shown in FIG. 1, has a fairly low partial pressure.
While physical bombardment with heavy ions can remove some fraction of the adsorbed species, the resulting etch rate is much slower for the AlN or AlGaN etch-stop layer than GaN. Oxygen (O2) chemistries can lead to micromasking due to the formation of non-volatile SiOx via reaction with Si in the etch chamber or carrier wafer, while SF6 chemistries tend to lead to pitting for higher SF6 concentrations. See Lee, supra.
With some limitations, these etch chemistries can enable selective etching of GaN with respect to an Al-containing etch-stop layer, but cannot selectively etch a high Al fraction AlxGa1-xN or AlN layer.
Given the high vapor pressure of AlCl3 (plotline 103) and GaCl3 (plotline 105), Sc-containing etch-stop layers are effective with respect to AlxGa1-xN of any composition, including AlN due to the low vapor pressures of the expected etch byproducts such as ScCl3 (plotline 101) and ScF3 (plotline 102) of Sc-containing layers such as ScAlN and ScGaN.
However, Sc-containing etch byproducts such as ScCl3 (plotline 101) and ScF3 (plotline 102) have partial pressures many orders of magnitude lower than InCl (plotline 106) and InCl3 (plotline 107), suggesting they will be more difficult to remove from the sample surface, leading to a more robust and larger reduction of the etch rate with use of a Sc-containing etch-stop layer such as ScAlN and ScGaN.
Historically, the primary deposition technique for ScAlN has been reactive sputtering. Highly c-axis oriented films with thicknesses of 1 μm have been demonstrated having a piezoelectric response up to five times higher than sputtered AlN. See G. Piazza, et al., “Piezoelectric aluminum nitride thin films for microelectromechanical systems,” MRS Bulletin, vol. 37, pp. 1051-1061, 11 2012; see also M. Akiyama, et al., “Enhancement of Piezoelectric Response in Scandium Aluminum Nitride Alloy Thin Films Prepared by Dual Reactive Cosputtering,” Adv. Mater., vol. 21, pp. 593-596, 2009.
However, incorporation of impurities, particularly oxygen, can degrade the quality of ScAlN. See M. A. Moram, et al., “The effect of oxygen incorporation in sputtered scandium nitride films,” Thin Solid Films, vol. 516, pp. 8569-8572, 2008 (“Moram 2008”). Growth of ScAlN by molecular beam epitaxy (MBE) on high quality hexagonal substrates has the potential for improved crystal quality and greatly reduced impurity incorporation suitable for high performance electronic, optoelectronic, and acoustoelectric devices. In addition, growth of ScGaN by MBE has recently been demonstrated. See S. M. Knoll, et al., “Defects in epitaxial ScGaN: Dislocations, stacking faults, and cubic inclusions,” Appl. Phys. Lett. 104, 101906 (2014).